Men who have sex with men (MSM) who use drugs are a high-risk population. Major risk behaviors include unprotected sexual activity and the use of both injected and non-injected drugs. These risk practices are of concern because they account for so many of the existing cases of HIV infection and recent epidemiological evidence implicates drug abuse in HIV seroconversion among MSM. Of particular concern are increasing reports of both injected and non-injected use of "club drugs" in MSM social venues. Club drugs include a number of previously studied substances such as methamphetamine and inhaled nitrites, as well as a number of relatively new and as yet unexamined substances such as ketamine and MDMA. All four drugs are widely available in gay bars, sex clubs, and bathhouses, and have been associated with both injection and sexual risk practices. However, research on any one of these drugs is quite limited and there have been no studies which examine the interrelationship between them, the mode and frequency of their use, and their specific relationship to high risk sexual activity among MSM. An understanding of the social, ecological and psychological factors that position MSM in potentially risk-laden drug and sexual transactions is essential to both acquiring an epidemiological understanding of the MSM population and the formulation of actionable prevention and service recommendation for MSMs. Using the contrasting MSM populations and environments in Boston and New York, this study will describe the population of men who have sex with men and use club drugs, including demographic characteristics, prevalence of drug and sexual risk behaviors, and involvement in high risk settings and social networks. The study will be conducted in three phases. In Phase I, a Community Assessment Process will identify venues in both cities and provide grounding in the environments and social groups in which MSM participate. In Phase II, a year-long ethnographic study of 150 MSM will employ participant-observation and in-depth semi-structured interviews in order to document patterns of initiation into drug and sex risk behavior, involvement in high risk sex and drug social networks, and the relationship between experiences of grief and loss associated with AIDS and drug and sex risk behaviors. Finally, in Phase III, using a venue-based probability sampling strategy, a survey instrument will be administered to 900 MSM (450 in each city) in order to develop estimates of prevalence of club drug use among men attending the sampled venues, and document correlates of drug use and the relationship between drug and sexual risk behaviors in the MSM population.